Ti-Doping-Induced Interstitial Cu and the Origin of Rattling Vibrations in Cu ₇ VSnS ₈ -Based Thermoelectrics

Elucidating the details of crystal structures, such as the occupancy of interstitial sites and atomic vibrations, is essential for understanding the electrical and thermal properties of solids. In this study, we investigated the detailed crystal structures and their temperature evolution for thermoelectric materials Cu 7 VSnS 8 and Cu 7 V 0.5 Ti 0.5 SnS 8 using multi-temperature synchrotron radiation single-crystal and powder X-ray diffraction at SPring-8, combined with analyses of the electron density maps reconstructed via the Maximum Entropy Method. Furthermore, the local coordination environments were identified using X-ray absorption spectroscopy at NanoTerasu. This study revealed the existence of Cu atoms at interstitial sites in the Ti-doped system and large atomic displacement parameters at specific Cu sites in both systems. The occupancy of interstitial Cu atoms was found to increase above 500 K, which induces an abrupt lattice expansion. Density functional theory calculations demonstrated that the large-amplitude vibration observed for the Cu atom along a specific crystallographic direction is attributed to weak Cu−S bonding. These findings provide a pathway to optimize thermoelectric properties through structural modification by introducing interstitial cations via doping or thermal treatment and by inducing large atomic vibrations through tuning bond strength in sulfide-based thermoelectric materials.